The present invention relates generally to inkjet printing devices, and more particularly to an inkjet printhead for thermal inkjet printing devices that reduces the number of interconnections providing electrical connection to the drop ejector heater resistors with out sacrificing reliability.
The art of inkjet printing technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines successfully employ inkjet technology for producing hard copy printed output. The basics of the technology has been disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Inkjet devices have also been described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices (R. C. Durbeck and S. Sherr, ed., Academic Press, San Diego, 1988, chapter 13).
A thermal inkjet printer for inkjet printing typically includes one or more translationally reciprocating print cartridges in which small drops of ink are ejected by a drop generator towards a medium upon which it is desired to place alphanumeric characters, graphics, or images. Such cartridges typically include a printhead having an orifice member or plate that has a plurality of small nozzles through which the ink drops are ejected. Beneath the nozzles are ink firing chambers, enclosures in which ink resides prior to ejection by an ink ejector through a nozzle. Ink is supplied to the ink firing chambers through ink channels that are in fluid communication with an ink reservoir, which may be contained in a reservoir portion of the print cartridge or in a separate ink container spaced apart from the printhead.
Ejection of an ink drop through a nozzle employed in a thermal inkjet printer is accomplished by quickly heating the volume of ink residing within the ink firing chamber with a selectively energizing electrical pulse to a heater resistor ink ejector positioned in the ink firing chamber. At the commencement of the heat energy output from the heater resistor, an ink vapor bubble nucleates at sites on the surface of the heater resistor or its protective layers. The rapid expansion of the ink vapor bubble forces the liquid ink through the nozzle. Once the electrical pulse ends and an ink drop is ejected, the ink firing chamber refills with ink from the ink channel and ink reservoir.
Following removal of electrical power from the heater resistor, the vapor bubble collapses in the firing chamber in a small but violent way. Components within the printhead in the vicinity of the vapor bubble collapse are susceptible to fluid mechanical stresses (cavitation) as the vapor bubble collapses, thereby allowing ink to crash into the ink firing chamber components. The heater resistor is particularly susceptible to damage from cavitation. One or more protective layers are typically disposed over the resistor and adjacent structures to protect the resistor from cavitation and from chemical attack by the ink. One protective layer in contact with the ink is a mechanically hard cavitation layer that provides protection from the cavitation wear of the collapsing ink. Another layer, a passivation layer, is typically placed between the cavitation layer and the heater resistor and its associated structures to provide protection from chemical attack. Thermal inkjet ink is chemically reactive, and prolonged exposure of the heater resistor and its electrical interconnections to the ink will result in a degradation and failure of the heater resistor and electrical conductors.
The heater resistors of a conventional inkjet printhead comprise a thin film resistive material disposed on an oxide layer of a semiconductor substrate. Electrical conductors are patterned onto the oxide layer and provide an electrical path to and from each thin film heater resistor. Since the number of electrical conductors can become large when a large number of heater resistors are employed in a high density (high DPI--dots per inch) printhead, various multiplexing techniques have been introduced to reduce the number of conductors needed to connect the heater resistors to circuitry disposed in the printer. See, for example, U.S. Pat. No. 5,541,629 "Printhead with Reduced Interconnections to a Printer" and U.S. Pat. No. 5,134,425, "Ohmic Heating Matrix". Each electrical conductor, despite its good conductivity, imparts an undesirable amount of resistance in the path of the heater resistor. This undesirable parasitic resistance uselessly dissipates a portion of the electrical energy which otherwise would be available to the heater resistor thereby wasting energy that could otherwise be employed in more efficiently ejecting ink drops. If the heater resistance is low, the magnitude of the current drawn to nucleate the ink vapor bubble will be relatively large resulting in the amount of energy wasted in the parasitic resistance of the electrical conductors being significant relative to that provided to the heater resistor. That is, if the ratio of resistances between that of the heater resistor and the parasitic resistance of the electrical conductors (and other components) is too small, the efficiency (and the temperature) of the printhead suffers with the wasted energy.
Most of the thermal inkjet printers available today use square heater resistors that have a resistance of 35 to 40 .OMEGA.. If it were possible to use resistors with higher values of resistance, the energy needed to nucleate an ink vapor bubble would be transmitted to the thin film heater resistor at a higher voltage and lower current. The energy wasted in the parasitic resistances would be reduced and the power supply that provides the power to the heater resistors could be made smaller and less expensive.
As users of inkjet printers have begun to desire finer detail in the printed output from a printer, the technology has been pushed into a higher resolution of ink drop placement on the medium. One of the common ways of measuring the resolution is the measurement of the maximum number of ink dots deposited in a selected dimension of the printed medium, commonly expressed as dots per-inch (DPI). The production of an increased DPI requires smaller drops. Smaller ink drops means a lowered drop weight and lowered drop volume for each drop. Production of low drop weight ink drops requires smaller structures in the printhead. So, designers of inkjet printheads are faced with the problem of more drop generators (with their associated heater resistors) disposed over a smaller area of printhead being operated at an increased frequency. In order to energize the greater number of smaller drop generators, an increased number of electrical conductors is required on a smaller area of printhead substrate real estate.
The aforementioned multiplexing techniques have helped reduce the total number of conductors necessary to energize the heater resistors. More improvement is needed, however, to reliably reach the high drop generator densities required for modern printing operations.